Efforts were made in the mid-1960s to advance the art of ladle steelmaking as a result of increasing costs and other factors in the steel industry. One effort culminated in the system known as vacuum arc degassing which utilizes three essential features: subjection to proper vacuum levels; gas stirring; and alternating current (hereafter "AC") arc heating at a time when the steel is subjected to the simultaneous effect of the aforesaid vacuum and gas stirring. The treatment of the steel by subjection to vacuum and gas stirring culminated in the process described in U.S. Pat. No. 3,236,635, whose disclosure is incorporated herein by reference. This process, though used extensively commercially by the assignee of this invention, and, in subsequent years, by others, had a time limitation due to the permissible temperature drop of the steel. The further feature of AC arc heating as exemplified in U.S. Pat. No. 3,501,289 eliminated the temperature drop limitation and permitted end point degassing, as well as providing discretionary time which could be used to carry out other treatment, or, simply, as a holding expedient required by production limitations.
Since vacuum arc degassing, (sometimes hereafter referred to as VAD) has been developed and proven, this process and other ladle steelmaking techniques have established themselves as the quickest, simplest, and most economical way to make carbon and low alloy steels.
In fact, ladle steelmaking is now one of the most popular steelmaking techniques in the market today. Yet during this highly competitive period of steelmaking it is essential that the steelmaker further reduce his ladle steelmaking costs. This can be accomplished by reducing time, energy, and space required for ladle steelmaking.
Shortening process time is essential to increase throughput, thereby reducing capital costs per ton treated and enabling the VAD to keep up with UHP and pneumatic furnaces. In ladle steelmaking, time equates to temperature loss, as all the time steel is held in the ladle it is losing heat. This heat loss can be made up by superheating in the melting furnace or arc heating in the ladle. Either technique consumes costly electrical energy which can be appreciably reduced by shortening VAD process time using the following new and unique techniques.
Shortening process time also permits use of a single process station, whereby degassing and arc heating can be simultaneously carried out without the logistic complexities of multi-station, single purpose installations which consume both more time and space in the melt shop.